Detection and classification of gamma ray emitters has attained heightened importance in the protection of vulnerable targets and populaces from special nuclear materials. Many fissionable special nuclear materials emit gamma rays, due to radioactive decay of the elements therein. However, many less harmful and non-fissionable materials also emit gamma rays. Therefore, it is desirable to be able to identify, and whenever possible, distinguish between the types of gamma ray emitters in an unknown material, possibly further concealed inside of a container or vehicle of some type, such as a car, van, cargo container, etc.
Many types of materials emit gamma rays that appear very close together in a gamma spectrum. Scintillator detectors use materials that emit bursts of light when gamma rays interact with the atoms in the scintillator material. The amount of light emitted can be used to identify the isotope that is emitting the gamma rays. Scintillator detectors may also be used to detect other types of radiation, such as alpha, beta, neutron and x-rays. High energy resolution scintillator detectors are useful for resolving closely spaced gamma ray lines in order to distinguish between different gamma-emitting radioisotopes.
Detection sensitivity for weak gamma ray sources and rapid unambiguous isotope identification is principally dependent on energy resolution, and is also enhanced by a high effective atomic number of the detector material. Generally, gamma ray detectors are characterized by their energy resolution. Resolution can be stated in absolute or relative terms. For consistency, all resolution terms are stated in relative terms herein. A common way of expressing detector resolution is with Full Width at Half Maximum (FWHM). This equates to the width of the gamma ray peak on a spectral graph at half of the highest point on the peak distribution.
The relative resolution of a detector may be calculated by taking the absolute resolution, usually reported in keV, dividing by the actual energy of the gamma ray also in keV, and multiplying by 100%. This results in a resolution reported in percentage at a specific gamma ray energy. The inorganic scintillator currently providing the highest energy resolution is LaBr3(Ce), about 2.6% at 662 keV, but it is highly hygroscopic, its growth is quite difficult and it possesses natural radioactivity due to the presence of primordial 138La that produces betas and gamma rays resulting in interference in the gamma ray spectra acquired with LaBr3(Ce). Therefore, it would be desirable to have a scintillator detector system that is capable of distinguishing between weak gamma ray sources that is more easily grown while still providing high energy resolution.
Europium-doped Strontium Iodide is a high light yield scintillator with excellent light yield proportionality and physical properties amenable to low cost, facile crystal growth, such as low melting point, unity distribution coefficient for the Eu dopant and modest anisotropy. However, its decay time of ˜1 microsecond results in difficulty in accurate pulse readout with traditional analog pulse shaping electronics. In addition, the overlap between the Eu2+ absorption and emission spectra, and the requirement for high Eu2+ doping to achieve the highest light yield crystals, results in lengthening of the effective decay times. Most commercial gamma spectroscopy radioisotope identifiers utilize direct numerical integration implemented with analog electronics.